How important is a GPU for modeling?
I will assume you are talking about an external GPU in this case, say a Nvidia GTX 660 etc..
Excerpt from the Manual

...GPU rendering makes it possible to use your graphics card for
rendering, instead of the CPU. This can speed up rendering, because
modern GPUs are quite designed to do a lot of number crunching...

In short, No, you do not need an external GPU. Not to get too technical but except for drawing and viewport stuff amongst some other things, much of Blender's core functionality relies very little on a fully fledged GPU.
For modeling in your case, you are perfectly fine with just a decent CPU and an onboard graphics card that supports OpenGL (which in these modern times is pretty much every one of them).
Having one would be nice though as this can greatly improve the viewport performance among other things but it is not a major requirement unless you are going to be doing very heavy or demanding production work.

Graphics card note: Blender runs on all OpenGL compliant cards,
although there are some issues with cheap onboard cards (Intel, Via).
We therefore can't officially support and guarantee that Blender works
fine on those systems.

You can see the system requirements for Blender here.